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Epidermis epi=above, upon
•keratinized stratified squamous epithelium
-Dead cells at the surface packed with tough protein called keratin
–Lacks blood vessels (avascular)
–Depends on the diffusion of nutrients from underlying connective tissue (in dermis)
–Sparse nerve endings for touch and pain
Cells of the Epidermis
•Undifferentiated cells that (once they divide)give rise to keratinocytes
•In deepest layer of epidermis (stratum basale)
•Occur only in stratum basale
•Synthesize (brown to black) pigment melanin that shields DNA from ultraviolet radiation
•Branched processes that spread among keratinocytes
•Everyone has the same number of melanocytes.
Melanocytes have branching processes that spread among the keratinocytes and continually shed melanin containing fragments from their tips. The keratinocytes phagocytize these fragments and accumulate melanin granules on the “sunny side” of the nucleus. Like a parasol, the pigment shields the DNA from UV radiation.
–Dendritic (Langerhans) cells (immune cells)
•Macrophages originating in bone marrow that guard against pathogens phag=eat
•Found in stratum spinosum and granulosum (just remember the picture for their location)
•Stand guard against toxins, microbes, and other pathogens that penetrate skin
•Epidermis has as many as 800 dendritic cells per square mm
Dendritic cells-Originate in bone marrow but migrate to the epidermis and epithelia of the oral cavity, esophagus, and vagina.
Layers of the Epidermis
–Up to 30 layers of dead, scaly, keratinized cells
–Also consists of dead keratinocytes and keratinocyte fragments
•Form durable surface layer–Surface cells flake off (exfoliate-exfoliating keratinocytes)
•Resistant to abrasion, penetration, and water loss(by absorption)
The keratin of the stratum corneum absorbs water and swells, but the deeper layers of the skin do not.
Stratum Lucidum Lucid=clear
–Seen only in thick skin
- •Thin translucent zone superficial
- to stratum granulosum
•Keratinocytes are densely packed with clear protein eleidin
•Cells have no nuclei or other organelles
•Zone has a pale, featureless appearance with indistinct boundaries
–Consists of three to five layers of flat keratinocytes
–The Keratinocytes of this layer contain coarse dark-staining keratohyalin granules
–More in thick skin than in thin skin
Stratum Spinosum(thickest layer)
–1AProduce more and more keratin filaments which causes cell to flatten; the higher up in this stratum, the flatter the cells appear
--2AThickest stratum in most skin–In thick skin, exceeded by stratum corneum
–3AConsists of several layers of keratinocytes
--3BKeratinocytes are firmly attached to each other by numerous desmosomes which partly account for epidermis’s toughness.
Histological fixatives shrink the keratinocytes causing them to pull away from each other, but they remain attached by desmosomes.
Desmosomes create bridges from cell to cell, giving, each cell a spiny appearance.
Epidermal Keratinocytes are also bound together by “tight junctions”, thus, essential for water retention
--4ADendritic cells found throughout this stratum
Numerous desmosomes and cell shrinkage produces spiny appearance
--5ANamed for artificial appearance (artifact) created in histological section
--6ADeepest cells remain capable of mitosis.
Cease dividing as they are pushed upward
(HINT: think of the picture in order to remember the make up of this layer)
–A single layer of cuboidal to low columnar stem cells and keratinocytes resting on the basement membrane
–Scattered among these are the melanocytes, tactile cells and stem cells.
–Stem cells of stratum basale divide and give rise to keratinocytes that migrate toward skin surface
–Replace lost epidermal cells
•connective tissue layer beneath the epidermis
–Ranges from 0.5 mm (eyelids) to 4 mm (palms, soles)
–Composed mainly of collagen with elastic fibers, reticular fibers, and fibroblasts
–Well supplied with blood vessels, sweat glands, sebaceous glands, and nerve endings
•superficial zone of dermis
–Thin zone of areolar tissue in and near the dermal papilla
–Allows for mobility of leukocytes and other defense cells should epidermis become broken
–Rich in small blood vessels
•deeper and much thicker layer of dermis
–Consists of dense, irregular connective tissue
–Stretch marks (striae): tears in the collagen fibers caused by stretching of the skin due to pregnancy or obesity
•most significant factor in skin color
–Produced by melanocytes
–Accumulate in the keratinocytes of stratum basale and stratum spinosum
- –Pheomelanin—a reddish yellow sulfur-containing pigment